How do some cells become brain cells and others become skin cells, when the dna in all the cells is exactly the same. in other words, if the instructions are exactly the same, how does one cell become a brain cell and another a skin cell?

This occurs through the process of cell specialization, where in some cells genes are active whereas in others they are not.

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.